Biofumigation for plant disease control – from the fundamentals to the farming system - Disease Control in Crops - page 182

Air

Incorporation

Soil

Isothiocyanate

Biofumigation

Days

Figure 9.3 Rapid decline in concentrations of 2-propenyl isothiocyanate in fi eld soil follow-
ing incorporation of pulverized green manure of high-GSL mustard biofumigant (from Gimsing &
Kirkegaard, 2006). Isothiocyanates are subject to various loss processes in soils which may reduce the
amounts available for biofumigation via reaction with the nucleophilic groups in soil-borne disease
organisms (modifi ed from Gimsing & Kirkegaard, 2008, with kind permission of Springer Science and
Business Media).

The rates and relative signifi cance of these processes are affected by the specifi c
side-chain structure of the ITC and soil properties such as organic matter content, water
content and temperature. ITCs are not as volatile as other synthetic soil fumigants, but
volatile losses, particularly for short-chained aliphatic ITCs with higher vapour pressure
(e.g. 2-propenyl ITC) can represent a signifi cant source of loss from soil. Rapid incorpo-
ration, soil covering techniques and surface watering can serve to reduce this loss, while
higher soil temperature will increase it (Price et al ., 2005). As a result of their hydropho-
bic nature, ITCs are rapidly sorbed to organic matter in soil and not to clay (Gimsing
et al ., 2008) and their pest control potential has been shown to be signifi cantly reduced in
soils with higher levels of organic matter (Matthiessen & Shackleton, 2005). Sorption is
likely to be greater for longer-chained aliphatic or aromatic ITCs which can reduce their
pesticidal effectiveness in headspace toxicity tests. (Matthiessen & Shackleton, 2005).
Studies comparing loss from sterile and non-sterile soils demonstrate that ITCs are also
degraded by microbes (Warton et al ., 2003; Rumberger & Marschner, 2003; Price et al .,
2005; Gimsing et al ., 2008) and persistence in sterile soil is often increased from hours
to days. Enhanced biodegradation by specialized organisms selected for by repeated use
of synthetic or plant-derived ITCs provides further evidence of microbial degradation
(Warton et al ., 2003). Increasing soil water content generally increases ITC concentration
and longevity in amended soil, presumably by facilitating more rapid GSL hydrolysis and
by increasing ITC in solution thereby reducing volatile losses. Temperature, pH and soil
texture tend to have less impact on ITC behavior in soil at the ranges normally experienced
in fi eld soils (Brown & Morra, 1997; Gimsing & Kirkegaard, 2008).